Potatoes in Colorado

It’s spring again and we once more have seed potatoes available at the Greenhouse. Seed potatoes are in fact just small tubers that are virus, pest, and disease free.

Potato Basics

Lavender Fields of Potatoes growing in their native Andes.

Potato growth occurs in five stages. First, the seed potatoes sprout buds and roots. This process, called ‘chitting’ can be done before planting indoors to ‘jump-start’ the process as bud creation is slowed by cold soil temperatures. In the second stage, photosynthesis begins to occur as leaves and branches develop. Below the soil surface, stolons or sideways stems develop. At the tips of these stems tubers begin to form in the third stage. This will often coincide with early flowering. In the fourth stage, the tubers (potatoes) will begin to bulk. Various factors including temperature, soil fertility, and soil moisture are critically important during the third (tuber set) and fourth (tuber growth) stages. The optimal temperature for tuber development is when soil temperatures are between 61-66ºF. Tuber formation slows when soil temperature rises above 68ºF and will halt when the soil temperature reaches 86ºF. Optimal potato development occurs when the soil moisture is maintained along with adequate soil aeration. Finally, in the last stage, the vines will begin to yellow and the growth of the tubers will slow and they will form skin. A plant will generally set 20 to 30 tubers of which 5 to 15 will reach maturity. Click here for more in-depth information.

Growing Potatoes on the Front Range

When to Plant

The front range can get quite hot in the summer and as you read above, potatoes don’t like temperatures above 90ºF. If you want good yields, you need to get your potatoes into the ground early. Seed potatoes should optimally be planted in soil temperatures around 50ºF and should not be planted in soils below 40ºF. Guides vary as to the recommended planting date. However, the earliest you should plant a seed potato is two weeks before the last anticipated hard freeze of 28ºF or two to four weeks before the last frost. In Boulder County, April 22nd has a 50% probability of being our last freeze, with the freeze range between April 8th (90% probability of a freeze) and May 7th (10% probability). Therefore, you should be planning on planting your seed potatoes in April, earlier or later depending on the season and your tolerance of risk. This is just a guideline and depends on the year; the real factor is not the air temperature but the soil temperature. To most accurately time your plantings, use a soil temperature probe, or follow the methods outlined below to jumpstart your potato plantings. ***Please note, in an earlier version of this article I had misread the freeze/frost probability and thus made an error in my start date calculation. The correction is as above.*** A late frost can blacken the newly emerged potato foliage but the potato will usually recover but for this reason some people choose to wait a few more weeks.

A Couple Methods to Jump-Start Your Potato Season

There are a few ways to get a jump-start on your potatoes and ‘cheat’ mother nature. One way is to ‘chit’ your potatoes indoors. In essence, you are performing the first stage of potato growth indoors instead of in the ground, allowing you to delay your planting without delaying your potatoes. To do this, examine your seed potatoes and find the end with the most eyes. Using an egg carton without its lid to stabilize, place the seed potato in one of the divots with the eyes pointing heavenward. Place the egg carton of potatoes in a cool window in bright, indirect light and allow the potatoes to form buds. Indirect light is key as you do not want weak, elongated buds. This process can take a few weeks. Once this has occurred, you can plant the potatoes outdoors in the warming spring soils.

Another way to plant early is by warming the soil for about a week pre-planting with black plastic. Leaving this plastic on the ground around your potato plants throughout spring will raise the soil temperatures and spur your potatoes to develop their roots and foliage more quickly. Keep in mind, that freezes may still occur and be prepared to protect your newly emerged potatoes with frost clothes or by using such devices as a Wall of Water. Using plastic changes the following planting recommendations, so please follow the instructions here.

Soil Preparation, Planting, and Maintenance

Potatoes grow best in fertile soils. The typical clay or sandy soils in Colorado need amending with composts in order to grow potatoes well. If you plan to plant your potatoes in the ground, space them 10-12″ apart in a row and your rows 24″ apart. Planting close together as described allows the foliage to shade the soil, maintaining cooler soil temperatures for longer into the season.

Sometimes the stolons (and potatoes) will begin to form along the surface of the soil. Photosynthesis will begin to occur on these exposed parts causing a greening and the production of the toxic solanine. To prevent this, you can mound soil around the base of the plant as it grows or spread straw around the base. Using straw or mulch can have the added benefit of shading the soil: maintaining cooler soil temperatures, reducing moisture loss, and preventing weed growth. This article contains a diagram to help you picture this process.

Be cautious when using any tools in the soil near your potato plants for weeding or mounding soil as the developing tubers are generally just inches from the surface.

Harvesting

In a home garden, it is possible to harvest early, just a few potatoes from a plant while leaving the plant in place. These smaller, immature potatoes are called ‘new potatoes’ in stores and are often quite flavorful. When the vines have yellowed and died, it is time to harvest your potatoes. Again, be careful not to damage your potatoes. Use a garden fork or hand trowel.

Growing Potatoes in a Container

Growing potatoes in a container can be quite rewarding as well. I came across a fine article on this method and am eager to test it myself this season.